The present invention relates to a local vacuum type electron beam welding process and apparatus and, more particularly, to a local vacuum type electron beam welding process and apparatus suitable for a relatively long member of large size of electroconductive material that is to be produced by welding a plurality of pieces along a relatively short welding line in a direction traversing the length direction thereof, and to the member produced by the apparatus or method.
An example of the above-mentioned relatively long electroconductive member of large size is a poloidal field coil for a nuclear fusion reactor system. The poloidal field coil is disposed out of a vacuum chamber of a fusion reactor to generate a magnetic field for enclosing plasma in the vacuum chamber. The coil has an annular shape of which the diameter is 10 m, for example. The coil consists of a plurality of parts each having thickness of several ten mm and width of several hundred mm. The parts are arranged in an annular configuration and joined by welding the parts along the width direction, thereby forming the coil.
The coil has a large size, however and, welding lines between the parts are relatively short. The coil is produced by repeating the welding along the relatively short welding lines many times.
Hitherto, the coil was produced by using a high pulse TIG welding method. However, the welding method caused problems such that the strength in welded portions is lower than one of the parent material and the electric resistance in the welded portions increases by the welding.
It is known that the above-mentioned problems are solved by using an electron beam welding method. However, an electron beam welding apparatus having a large vacuum chamber which can contain such a large sized coil reaching to the diameter of 10 m is not available.
On the other hand, a local vacuum type electron beam welding apparatus is known which welds a member with electron beams by placing partially the member including a part to be welded into a vacuum chamber or by forming a vacuum portion around a part of the member. An example of prior arts of such a welding method or apparatus is disclosed in U.S. Pat. No. 4,080,526 filed Mar. 21, 1978.
An electron beam machining apparatus disclosed in the US patent comprises upper and lower fixed plates disposed on upper and lower sides of a member to be machined, respectively, to form upper and lower vacuum chambers, sealing members disposed between the member to be machined and the upper and lower plates around a machining or welding line, and a movable plate movably disposed on the upper fixed plate mounting thereon an electron gun and forming a movable vacuum chamber communicating with the upper vacuum chamber. The upper fixed member has a narrow groove extending the machining or welding line and a belt fitted in the groove. The belt is removed between forward and backward sides of the movable vacuum chamber to communicate the movable vacuum chamber with the upper vacuum chamber, and the belt is removed at the forward side of the movable vacuum chamber and fitted at the backward side to the groove as the movable plate moves. Thus, the belt is removed only at the movable vacuum chamber.
In the apparatus, a movement of the electron gun in a perpendicular direction to the machining or welding line is limited to within a width of the narrow groove, so that precise positioning between the machining or welding line and the upper plate is necessary.
Therefore, the apparatus is suitable for machining or welding of a member having a relatively long machining or welding line, but is not suitable to applying a member of large scale having a plurality of relatively short welding or machining line because the precise positioning is necessary many times.
Further, in order to seal the machining or welding or line ends, sealing members and pressing members pressing the sealing members are mounted at the end portions, so that it is difficult to weld the end portions to an extent that the weld is nearly the same as other portions.
Another example of the local vacuum type electron beam welding apparatus is disclosed in Japanese Patent Laid-Open No. 59-225891/1984 laid-opened Dec. 18, 1984. The apparatus comprises local vacuum chambers between which a member to be welded is sandwiched, and it is characterized by a sealing means between the local vacuum chambers. The sealing means comprises helium gas chambers disposed on both outsides of the vacuum chambers and containing therein helium gas of slightly higher pressure than the atmospheric pressure, and it is constructed so that the helium gas leaks into the interior of the vacuum chambers through gaps between the member to be welded and the local vacuum chambers. Thus, the electron beam welding is effected in an atmosphere favorable to welding.